A tooth that has lost its function due to decaying or accident can be restored by, for example, securing, to the tooth, a metallic or ceramic restorative for a crown and an inlay. An adhesive called dental cement is used for securing the crown restorative to the tooth. As the dental cement, there is usually widely used a resin cement and a fluoroaluminosilicate cement (generally called glass ionomer cement).
To adhere the crown restorative to the tooth by using a dental cement, in general, the dental cement is applied in a slightly excess amount to the crown restorative, and the crown restorative is brought into pressed contact with the tooth. Due to the pressed contact, an excess of the dental cement swells beyond the junction portion between the tooth and the crown restorative, which is called marginal portion, and the excess of cement that has swollen is removed. Therefore, the dental cement is offered as a highly fluidizing paste-like composition so that it can be easily applied onto the crown restorative and that an excess amount thereof suitably comes out from the marginal portion. Unless the excess of cement is completely removed, further, the cement that has swollen and cured may not only deteriorate aesthetic appearance but also hurt the tissues in the oral cavity.
Usually, the excess of cement is removed by using an explorer or the like. However, it is difficult to remove the cement by using the explorer if it is in a highly fluidized state without at all being cured. Therefore, the excess of cement is removed in a state where the cement is completely cured or is cured to some extent to lose the fluidity (half-cured state).
Among the dental cements, the resin cement comprises a radically polymerizable monomer and an inorganic or organic filler, and is cured upon being polymerized with a radical polymerization initiator. Upon being blended with an acidic group-containing radically polymerizable monomer, the resin cement firmly adheres to the tooth and to various metals.
The resin cement blended chiefly with an inorganic filler exhibits particularly excellent mechanical strength and durability. When the cement featuring a high mechanical strength is used, the excess of cement can be removed with difficulty after it has been completely cured. Therefore, the excess of cement is removed while it is in a half-cured state. However, the cement is blended with a polymerization initiator in an amount enough for completely curing the cement. Accordingly, the cement stays in a half-cured state for only a short period of time, leaving a difficulty in regard to the timing for removal.
When the resin cement is blended chiefly with an organic filler, on the other hand, the cured body of the cement turns into an elastic material which still can be removed with difficulty by using the explorer after it has been completely cured. Therefore, the excess of cement is removed in a half-cured state. Like when it is blended with an inorganic filler, however, the cement stays in the half-cured state for only a short period of time, leaving a difficulty in regard to the timing for removing the excess of cement.
It becomes very difficult to remove the excess of cement from the portions other than the desired portion once it is completely cured because the resin cement is strongly adhered to the tooth and to the crown restorative irrespective of whether the inorganic filler is contained or the organic filler is contained.
Japanese Unexamined Patent Publication (Kokai) No.9-67222 discloses a technology for delaying the curing time by adding a polymerization inhibitor. In this case, however, the polymerization reaction proceeds at one time after the polymerization inhibitor is all spent out for delaying the polymerization time. Therefore, a fully satisfactory result is not obtained from the standpoint of elongating the timing for removing the excess of cement. There may further arouse such a problem that the strength of adhesion to the tooth decreases with an increase in the amount of the inhibitor.
In order to cure the resin cement, further, there is usually used a system comprising a tertiary amine and an organic peroxide such as benzoyl peroxide as a chemical polymerization initiator. The chemical polymerization initiator comprising this system works to reliably accomplish the curing and establishes safety for the living body. When this system is used, however, there arouses a problem in that the cured body of cement is colored in yellow. The degree of yellow color usually tends to be stronger than the yellow color of natural teeth. After the crown restorative is secured, therefore, the marginal portion of cement becomes so conspicuous as to impair aesthetic appearance.
The above problem of coloring can be solved to some extent by using “a catalyst system of a combination of a pyrimidine trione derivative, a metal compound and an organohalogen compound” disclosed in Japanese Unexamined Patent Publication (Kokai) No. 63-216811. Even when this catalyst system is used, however, there still remains unsolved the problem in that the time margin is not sufficient for removing the excess of cement.
On the other hand, the glass ionomer cement comprises a powder that elutes multiply charged metal ions and an aqueous solution of a polycarboxylic acid, and cures when the powder and the aqueous solution are mixed together as the multiply charged metal ions and the polymer (polycarboxylic acid) undergo the chelate crosslinking. The above cement is easy to handle since it does not usually require the pretreatment for the tooth and offers such a feature that the excess of cement can be favorably removed. Further, the curing mechanism of the glass ionomer cement does not cause a discoloration of the cured body.
Here, however, the reason that the glass ionomer cement permits the excess of cement to be favorably removed is that it is easily collapsed at the time when it is being removed by using the explorer, since the mechanical strength of the completely cure body of cement is smaller than that of the resin cement. Accordingly, while offering the advantage as described above, the glass ionomer cement involves a problem in regard to the durability (reliability) of the cement itself. Another problem of the glass ionomer cement is that when it comes in contact with water such as saliva at the time when it is being curing, the properties such as mechanical strength and the like are deteriorated.
In order to solve the problems possessed by the glass ionomer cement, there has recently been developed and placed in the market a dental cement called resin-reinforced glass ionomer blended with a radically polymerizable monomer and a chemical polymerization initiator in addition to the polycarboxylic acid. In the above cement, the radically polymerizable monomer is polymerized during the curing based on the chelate crosslinking, and the polymer of the radically polymerizable monomer is made present in the cured body that is obtained thereby to improve the mechanical strength that was the defect of the glass ionomer.
Even by using the resin-reinformed glass ionomer, however, the cured body still chiefly comprises the polycarboxylic acid and the chelate (ionomer) of a multiply charged metal-on. Therefore, the mechanical strength is still inferior to that of the resin cement comprising chiefly a polymer of a radically polymerizable monomer, and the reliability is not improved to a sufficient degree.
By using the resin cement having a large strength of adhesion to the tooth and a large mechanical strength, the problem remains with respect to removing the excess of resin and the discoloration of the cured body. By using the glass ionomer cement (containing the resin-reinforced glass ionomer) which permits the excess of cement to be easily removed and exhibiting almost no discoloration), on the other hand, the problem remains with respect to the mechanical strength.